1. Field of Invention The present invention is related to a wireless communication receiver and in particular impulse noise mitigation.
2. Description of Related Art
Noise and in particular impulse noise, which is generated in short bursts, can be disruptive to data (broadcasts) that are processed through an analog receiver and translated into a digital format to produce a quality output as one might experience in a received radio transmission. The impulse noise can be caused by many modern day sources in which ignition systems and domestic appliances represent a couple of sources. Elimination or mitigation of impulse noise is essential to a clear reproduction of the received analog signal into a digital signal format.
US Patent Application Publication 2010/0054150 (Oksman et al.) is directed to a method and system in which impulse noise is monitored and noise protection parameters are adjusted. In US Patent Application Publication 2009/0323903 (Cioffi et al.) a method and apparatus is directed to monitoring and adjusting noise abatement in a DSL link. In US Patent Application Publication 2009/0168929 (Liu et al.) a method and apparatus is directed to an adaptive impulse noise detection and suppression. In US Patent Application Publication 2003/0099287 (Arambepola) a method and apparatus is directed to detecting impulse noise in COFDM modulated TV signals. U.S. Pat. No. 7,676,046 B1 (Nelson et al.) is directed to a method of removing noise and interference from a signal by calculating a time-frequency domain of the signal and modifying each instantaneous frequency. U.S. Pat. No. 7,630,448 B2 (Zhidkov) is directed to a method to reduce noise in a multiple carrier modulated signal by estimating impulse noise and removing the noise as a function of the estimated impulse noise. U.S. Pat. No. 7,573,966 B2 (Kim et al.) is directed to a signal conditioning filter and a signal integrity unit to address equalization and noise filtering to improve signal fidelity. In U.S. Pat. No. 7,558,337 B2 (Ma et al.) a method and apparatus is directed to signal processing to mitigate impulse noise. In U.S. Pat. No. 7,499,497 B2 (Huang et al.) a method and apparatus is directed to suppression of impulse noise in an OFDM system. U.S. Pat. No. 7,302,240 B2 (Koga et al.) is directed to a communication apparatus that has an ADC to convert an analog signal to a digital signal before applying an noise detector.
U.S. Pat. No. 7,139,338 B2 (Wilson et al.) is directed to a receiver with a filter and an impulse response from which a controller adapts the impulse response to the filter. U.S. Pat. No. 7,035,361 B2 (Kim et al.) is directed to a signal conditioning filter and a signal integrity unit to address coupled problems of equalization and noise filtering. In U.S. Pat. No. 7,016,739 B2 (Bange et al.) a system and method is directed to removing narrowband from an input signal in which notch frequencies of notch filters are adjusted in accordance with a detected noise spectrum. In U.S. Pat. No. 6,920,194 B2 (Stopler et al.) a method and system is directed to correcting impulse noise present on an input signal. U.S. Pat. No. 6,795,559 B1 (Taura et al.) is directed to an impulse noise reducer, which detects and smoothes impulse noise on an audio signal. U.S. Pat. No. 6,647,070 B1 (Shalvi et al.) is directed to a method and apparatus for combating impulse noise in digital communication channels. U.S. Pat. No. 6,385,261 B1 (Tsuji et al.) is directed to an impulse noise detector an noise reduction system in an audio signal. U.S. Pat. No. 5,410,264 (Lechleider) is directed to an impulse noise canceller, which recognizes, locates and cancels impulse noise on an incoming signal. U.S. Pat. No. 5,226,057 (Boren) is directed to adaptive digital notch filters for use with RF receivers to reduce interference. U.S. Pat. No. 4,703,447 (Lake, Jr.) is directed to a mixer controlled variable passband finite impulse response filter. U.S. Pat. No. 4,703,447 (Lake, Jr.) is directed to a mixer controlled variable passband finite impulse response filter.
A primary purpose of a receiving tuner is to select a particular channel of interest and convert that frequency band to a baseband for digital signal processing. Shown in FIG. 1 of prior art an output 11 of a tuner 10 is processed through an analog to digital converter (ADC) 12 to translate the analog output 11 of the tuner into a digital time domain waveform. Mitigation of sudden spikes in the time domain waveform, which are caused by impulse noise, prevent an accurate demodulation 16 of the digital signal produced by the ADC 12. The output 13 of the ADC12 is applied to an impulse noise mitigation circuit 14 and the output 15 of the impulse noise mitigation circuit is connected to a demodulator 16.
Shown in FIG. 2 of prior art is an expansion of the impulse noise mitigation 14 for time domain noise mitigation for impulse noise interference detection. An output of a magnitude function 20 is compared to a threshold using a standard comparator 22. the output 23 of the comparator 22 is used as an impulse noise flag by the suppressor circuit 24. The output of the suppressor circuit 15 is connected to the demodulator 16. The detection threshold of the comparator 22 can either be a fixed predetermined value or the detection threshold can be dynamically calculated based on the output 21 of the magnitude function. The suppressor circuit 24 either clips the samples of the digital signal that are found to be impulse noise in the comparator or nulls out the corrupted samples of the digital signal caused by impulse noise.
A shortcoming of the time domain method of impulse noise mitigation, shown in FIG. 2, is the inability to detect the presence of impulse noise under normal or relatively high carrier to interference ratio. The impulse noise can often be buried under the average envelop of the desired signal.
FIG. 3 is an impulse noise mitigation scheme of prior art in the frequency domain. The output of the ADC 13, shown in FIG. 1, is connected to a high pass filter 30 and the output 31 of the high pass filter is connected to a magnitude function 32. The output of the magnitude function 33 is applied to a comparator circuit 34 having a detection threshold control. The output 35 of the comparator is connected to a suppressor circuit 36, which connects 15 back to the demodulator shown in FIG. 1
In the scheme shown in FIG. 3 the detection threshold can be fixed to a predetermined value or dynamically adjusted based on the output 33 of the magnitude function 32. The suppressor circuit 36 either clips the signal samples that are determined to be impulse noise or nulls out the corrupted samples.
The main drawback of the frequency domain method shown in FIG. 3 is the inability to detect the presence of impulse noise under out of band interferers, especially adjacent interferers, where the signal at the output of the magnitude function 33 will contain the energy of both the impulse noise and the interferers thus making the detection of impulse noise unreliable.